Density-Related Gas Separation

Diffusion of gases

That's what I generally thought would counter separation, assuming that you had a perfect insulator around your container to prevent any convective mixing. But is there a separation process that diffusion is countering? And would you end up with a net gradient?

Thanks, I think we're getting somewhere, but just to play a little more with the practical and theoretical:

The practical question is whether using helium as a tracer gas in a relatively quiescent-minimal flow, long and largely horizontal, piping system would have any valid separation questions. The tracer gas would be introducted at one end and measured well downstream as a function of time.

I believe, from a mixing perspective, that the impact of helium's lower molecular weight relative to an air or nitrogen or steam fluid in which it might be mixed is a negligible issue. I think this is effectively your point also.

The theoretical question is how to "prove it" if pushed.

Does hot air rise because of increased molecular kinetic energy? Or the associated density change? How do gas molecules respond to gravity at the molecular level anyway? I've never seen an equation for this. I'm just trying to figure our how to use (as an absurd example) a isolated "long vertical pipe" math model to demonstrate what might be the otherwise obvious.

It is somewhat difficult to go from what is actually happening at the atomic/molecular level to what we observe in the real world.

In the real world, you can take a thermometer and measure the temperature of a gas. However, at the molecular level it more complicated. The molecules are not all at the "temperature" that you just measured. To simplify, I'll use the term thermal energy to describe the "temperature" of the individual molecules. Some molecules will have a lot of thermal energy, some will have very little and their thermal energy state constantly changes, as energy is transferred from the walls of the container and between the molecules. What you measure as temperature is a reflection of the average thermal energy of the molecules.

Ok, back to the real world. When you heat a gas that is allowed to freely expand, the volume increases approximating V1/T1=V2/T2 where T is the temperature in Kelvin and V is the volume. So, if you have 1 liter of gas at 20C (293 K) and you raise the temperature to 313C (586K), you will now have 2 liters.

The mass of the gas is constant, but the volume is higher at higher temperatures. Since density is mass/V volume, hot air is less dense than cold. A hot air balloon rises because it's total density is lower than the surrounding air, just like a cork floats on water.

Gases can form layers if a lighter gas is slowly introduced over a less dense one (or vice versa). However, once mixed they will not spontaneously separate.

Tad

"...using helium as a tracer gas..."

All well and good when you know there's no naturally occurring He in the area to contaminate the results. However, He is relatively difficult to detect, being inert and all. More oftern refrigerant gases ("Freons") are used because they are not naturally occurring, are easy to detect, and diffuse through the pore spaces in soil very readily. There are some other gases that are used also, and they have similar characteristics.

It's pretty iffy that this tracer gas test would actually happen, but Varian advertised a sensitive helium detector. As you noted, we could very well have at least an atmospheric helium background potential to consider. Main concern about refrigerant gases, I was thinking, would be residual deposits? And stability at 500F?

The helium, at this layer is not so much naturally occuring also helium has a few other properties, may be a bit matched by only H₂ but H₂ is hazardous.

The main property is to rapidly pass through the smallest pores (smallest molecular radius) and not trying to combine with anything available in the pore (inert gas hence no chemical reaction)

Wiki has given an article on this check

http://en.wikipedia.org/wiki/Helium_mass_spectrometer

or this provides the details

http://www.atcinc.net/tracer-gas.asp

Not here - The hydorgen Ion is highly reactive and ma start reacting before it gets out of pore.

This is of course off the main OP but since we are at it hence enclosing.

Good discussion, and good links.

Thanks

You know there is a rate of diffusion - however can not help with mathematics, any way that depends on so many factors temperature, pressure, the gasses, etc - I am purely going by the memory (more than 30 years old so difficult now to get the equations or values, can only give the concept)

The rate of diffusion decides the time for the gas to spread over in another gas and make an unifrorm mixture.

If the gas because of what ever reason moves faster than this, it does not diffuse immediately, but in a more cone shape, the angle of the cone will be depending on the velocity of diffusion and the velocity of gas.

Put a ink drop (now not much available in market, but was there in our time) in water and it spreads in some time, not immediately.

However drop it from some height (if the drop does not break, you may see the cone formation)

This is waht happens with the hot air draft, the time to diffuse the hot air outwards is slower than the density difference and hence it rises up (also spreading of course) - I think I am out of my field - a meterologist may explain it better -

When you put He in the tupe - if it is vertical one - it will rise to top, if tube is short and remain for some times before it gets mixed.

Still the concentration will be a bit higher on the top.

The diffusion is on molecular level only

A molecule has a velocity component (that is random in a gas) and a net force of gravity. The random Velocity component is the diffuser and the gravity is the separator. Sorry not making a vector diagram, but you can just imagine it.

As the gravity moves the He to top, imagine all of the there.

At the boundary plane however, there will be molecules, that will have the velocity down wards and some Bromine, whose velocit will be upwards, so the boundary becomes hazy.

These gases , will slowly diffuse more and more.

If the column is sufficiently long, wou may have some partial separation, but no layers , at the center of layeres you may have very concentrated gas.

Once gases escape their bonding ability begins to interact with everything available, if there are some things not detected as present then there is the problem. In such ways that China makes Hydrogen gas from Aluminum much is inefficient in their process so that AL escaping into the atmosphere creates a buffer for more interaction of H and O but entraps CO² as a result making another complex problem in the atmosphere which results in Global Warming.

It has been determined that Aluminum Clusters reacting with water can produce Hydrogen Gas at room temperature. The reaction does not need heat or energy as a trigger, no traditional technique for splitting water to produce hydrogen is necessary. First Aluminum Clusters are synthesized, then they generate Hydrogen on demand without the need to store it and the amazing part of this is that Aluminum Clusters can be reused over & over by removal of the Hydroxyl Group (OH-) that remains attached to the Aluminum Clusters after they generate Hydrogen.

Exploding CO² is what I fear if products were made containing more CO² as a means to find uses for CO² that is if those products ended up in the Pacific Ocean and then dropped to the Oceanic Depths to become compressed volumes of CO² and then this compressed mass enters the Ring of Fire surrounding the Pacific Ocean and explodes causing Tsunami's and Earthquakes.

This is a tip off because of Aluminum Clusters possibly being within the atmosphere from the reactions in the Hydrogen Reactors that China has leaks and discharges into their atmosphere and ultimately the problem escalates to Global Warming.

CO² in plastics sounds firm of a concept creating many types of plastic products, but suppose these plastics become submerged, at super depths under the ocean where pressures are tremendous plastics can become compressed so then what happens to the CO² once it is compressed beneath the oceans? I am sure tons of it would need to be compressed to make a big difference. A example is a - foam coffee cups shrunk in size taken below the the Arctic Ocean depths (here: http://www.nytimes.com/2008/03/25/science/25cups.html).

The Ring of Fire is surrounding the Pacific Ocean (The Aleutian Islands, located off of Alaska, are in the Pacific Ring of Fire. IMAGE, Wikipedia Link), It is a volcanic region which when a pocket of CO² beneath the bottom surface has expanded and come upwards to encounter the volcanic region it become explodable. The situation causes tsunami's and earthquakes primarily. The concern I am making is that consumer good can become more subject to being thrown into the Pacific Ocean and therefore if they sink below into the depths of the oceanic waters they can become compressed, if then this CO² is a major part of the plastic being made which consumers throw away is compressed there is more in a given volume which would be subject to being consumed by the Ring of Fire and that is the problem. We just can't make these plastics with the consumer in mind and have to make them only with controlled applications like maybe wall insulation which is removable inside the hulls of buildings beneath the oceans, water purification plant construction, Monorail Construction, and the like not for consumer access goods like coats/jackets, cups, plates, furniture, automobile parts. Only controlled products where government manages the item. If we do not do this then the scenario plays out its course and earthquakes and tsunami's occur. The 'New York Times Article' simply puts it into perspective regarding the depths and pressures which would be exerted onto anything really that goes that depth it is not limited to foam cups as the article shows what happens to foam cups that have been subjected to depths of the ocean. My comment ... "At sea level, atmospheric pressure is 14.7 pounds per square inch. The deeper the dive, the greater the water pressure. At the resting place of the Titanic, more than two miles down, the pressure is 2.8 tons per square inch." Very deep water covers some 65 percent of the surface of this planet. If we neglect this problem then years into trying the products of CO² cause a abundance like the garbage dump already present above Hawaii circulating which has no destiny yet to be cleaned up. The more compressed CO² the more likened to a trapped pocket of CO² gas below the Oceanic Floor and the more likened to a similar explosion of that CO² gas volume at that depth it is a serious problem, See Video.

The kinetic energy of the heavy molecules and of the light helium atoms tends to, on average, become the same as they mix. The helium atoms, on average, move faster, and, of course, there is a range of velocities. Under usual circumstances, the helium will slowly diffuse, through gasses, liquids, even solids; that's why helium balloons go flat. At low pressures, as at the top of our atmosphere, where an atom can go a long distance before hitting another particle, the fastest helium atoms can escape gravity and be lost in space. This should not be a problem in your pipe.